Resistance welding is a method of joining metals together by making use of heat developed by an electric current, the metals being part of the electrical circuit. Resistance welding techniques are often used to attach small parts, such as nuts, to larger bases, such as sheet metal, and have been described, for example, in U.S. Pat. No. 4,020,316 to Schaft et al. and U.S. Pat. No. 4,789,768 to Tobita et al. The base is located between a pair of relatively movable welding electrodes, one of which has a retractable guide pin that holds the nut. The guide pin is then depressed so that the nut comes into contact with the face of the opposite electrode. Welding current is then passed through the nut and the base to weld the nut to the base. The guide pin holding the nut in place accordingly should be electrically insulated, in order to prevent arcing between the nut and the guide pin. The guide pin must also possess excellent wear resistance characteristics and sufficient mechanical strength to prevent chipping and breaking.
Prior art guide pins comprise metallic cores insulated by an alumina (Al.sub.2 O.sub.3), titania (TiO.sub.2), or an alumina-titania coating. Such coatings have been formed by an oxidizing heat treatment or by flame spraying. These types of guide pins have several disadvantages. For example, they exhibit low overall core hardness and hence low toughness and mechanical strength. The insulation films, especially those formed by flame spray coating, peel and chip easily due to poor bonding to the substrate, and thus results in pins that must be replaced often. The relatively low insulation film hardness results in poor wear resistance, and the inherently high levels of porosity result in deterioration of the film strength. The high surface roughness of these pins also results in poor nut slidability.
In response to the above drawbacks and disadvantages, all-ceramic pins have been developed such as set forth in U.S. Pat. No. 5,259,675 to Ichikawa et al. This patent discloses an all-ceramic guide pin having at least some surfaces that are microscopically gently curved, in order to improve nut slidability. Such all-ceramic pins are more expensive to manufacture compared with metallic pins (originally about twenty times more expensive) and those manufactured in accordance with Ichikawa et al. as above are still about two to four times more expensive than metallic pins, thereby making them impractical. Another major disadvantage of all-ceramic pins is the inherent brittleness of the ceramics used in their manufacture.
Accordingly, there remains a need for inexpensive, insulated guide pins with a high level of core hardness and thus high toughness and mechanical strength. The insulative coating or film should be tenaciously well bonded to the substrate and possess high hardness, and thus exhibit excellent wear resistance. In addition, the insulating surface needs to be smooth to facilitate nut slidability.